Our long-term goal is to elucidate the role(s) of insulin-degrading processes in health and in the etiology of diabetes mellitus. Insulin in vitro is degraded in a two-step sequential process: first, insulin is split by glutathione-insulin tranyshydrogenase (GIT) into A and B chains, which are further degraded by a second enzyme system, A/B chains protease(s). Six GITs from four different organs and three different species have been purified. GIT is ubiquitous, its action is probably the first and rate-controlling step in the disposition of insulin, and its concentration (as protein) in liver is under feedback control by the blood insulin level. Almost nothing is known about insulin A/B chains degrading protease(s). Specifically, our objectives are to continue investigations on the structure of GIT, to identify the regulatory mechanisms over the activity of GIT, and to examine the possible physiologic function(s) of GIT and A/B chains protease(s). Work will be carried out to define the biochemical mechanism for the induction of GIT by insulin, to determine the relationship between the insulin metabolic process and the cellular response to insulin, and to purify and characterize the A/B chains protease(s). Human circulating leukocytes contain GIT. Studies, using circulating leukocytes, to compare the insulin degradation in the normal, diabetic, prediabetic, obese, acromagalic and hypopituitary subjects are also proposed. BIBLIOGRAPHIC REFERENCES: P.T. Varandani, M.A. Nafz, and M.L. Chandler: Interaction of Insulin Analogs, Glucagon, Growth Hormone, Vasopressin, Oxytocin, and Scrambled Forms of Ribonuclease and Lysozyme with Glutathione-Insulin Transhydrogenase (Thiol: Protein-Disulfide Oxidoreductase): Dependence upon Conformation. Biochemistry, 14, 2115-2120, 1975. P.T. Varandani and M.A. Nafz: Insulin Degradation XVI. Evidence for the Sequential Degradative Pathway in Isolated Liver Cells. Diabetes, 25, 173-179, 1976.